Fishing Size Limits: How Fish are Gaming the System

A recent story in Nature describes a debate about whether fishing is causing fish to reproduce at smaller sizes. The debate is fueled by observations of shrinking average size in all kinds of fished populations, from lobster to swordfish.

The basic idea behind a lot of fisheries management is that we shouldn’t take too many juveniles in the fishery, because then there won’t be enough to grow up and reproduce. As a result many fisheries operate under size limits, and naturally (when the limits are enforced) the harvest is made up of mostly large fish. We now know, however, that in many species, large fish are disproportionately prodigious in their reproductive output – for example, a female cod that is 120 cm long could produce up to 13 times the number of eggs of a cod that is half as long. So it’s time to revisit the wisdom of size limits.

The question is whether fishing – by focusing on large fish – is changing the genetics of fish populations so that they spawn at smaller sizes and invest more energy in that than in growing to large sizes.

There is some evidence that fishing can change the genetics of fish populations. In one lab experiment, scientists harvested 90% of the fish from each of 6 populations. Only the largest fish were taken from two populations; only the smallest from another two populations; and in the final two fish were taken at random. After only four generations, fish in the populations that had lost all their big fish were only about one-third the average weight of those in the random-catch group.

This proves that evolution can occur when selection pressure is very strong, but it’s unclear what the implications are for fishery management.

Size limits in, say, a grouper fishery, may exert strong selection pressure for fish that mature earlier. After all, only those that mature quickly will have a chance to breed and pass on their genes. But size limits could also be selecting for fish that simply grow really fast. In this case, size limits might be making fish populations more productive.

Nature is Smarter Than We Are

Of course, the answer to this debate may not be either/or, shrinking fish or more productive ones. In nature, all the factors that can influence fish size, age at maturity, and productivity are operating at once. And there is a good reason why fish populations show a lot of overall variation, including growth rate and size at first maturity. It’s an uncertain world, even without fishermen, so it pays a species to hedge its evolutionary bets. When conditions favor fast growth and early maturity, at least some portion of the population is prepared to respond.

Given all that, the most reasonable approach to fishery management is probably not to stay with size limits or eliminate them. Rather, we should adopt management measures that protect all of the size classes of fish populations, since they are all important in some way. Juveniles, for example, need to mature and carry out their ecological roles as food for other species. Adults need to spawn and carry out their ecological roles as predators or ecological engineers. And big adults need to spawn and carry out their ecological roles as predators of larger organisms, which can sometimes help keep an entire ecosystem in balance.

In the end, management that succeeds in maintaining a diversity of size classes will likely result in more robust fisheries over time, as well as in healthier, more resilient ecosystems.

Marine protected areas, or MPAs, are essential elements of this kind of management, since it is in these no-take reserves that the “natural” size structure of a fish population can manifest itself. Once we know what that structure looks like, we can then manage toward it. For example, we can secure habitat that protects a certain life stage of a species, if it’s in a depleted state. Catch limits and other ways of managing overall fishing mortality are also important elements, of course, as are tools to ensure that goals are achieved and limits respected.

The saga of the incredible shrinking fish says, in the end, that we can learn a lot from nature about how to set the right goals for fisheries. The natural attributes of fish populations have served them in good stead for millions of years, through all kinds of adversity. If management is geared toward preserving those attributes, fish populations are likely to be able to withstand fisheries -- and many other kinds of impacts -- too.